1. Field of the Invention
The present invention relates to a heater and, more particularly, to a heater for a thermionic cathode.
2. Description of the Prior Art
Thermionic emission of electrons is a well-known phenomenon utilized to generate an electron beam. The emitted electrons can be shaped by known techniques into beams of various shapes for performing many useful functions. For example, U.S. Pat. Nos. 3,702,412; 3,745,396; 3,769,600 and 3,780,334 disclose apparatus in which elongated cathodes are heated by electric resistance.
U.S. Pat. No. 4,382,186 (which has been disclaimed in favor of U.S. Pat. No. 4,446,373 assigned to the assignee of the present invention) discloses an intense, fine-line, strip electron beam that can be used in many applications. It is particularly important in such applications to provide the electron beam at a predetermined location with predetermined dimensions. However, the magnetic fields associated with the generation and focussing of the electron beam, particularly the field set up by the electrical cathode heater, prevent accurate focussing and placement of the beam under many operating conditions.